I too agree with your hypothesis that correct mating of the contact surfaces of the delivery valve would be needed to ensure fuel delivery pressure and timing.i would have assumed the copper crush washer would take care of this.but now im doubting.i do know if you over torque the delivery valve in an attempt to obtain a better seal you will risk distorting the aluminum IP housing.if this happens it may be impossible to get a good seal

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michael cole 1980 300SD gone now but not forgotten,87tdt,90 300te 4matic 95 e320 wagon